This invention relates to hydrosteering systems, for motor vehicles, of the type providing a reaction to manual forces applied to the hand steering wheel.
For successful steering operation, the driver of a vehicle must be familiar with the limitations imposed by frictional forces between the vehicle tires and the road during vehicle travel. Only then will the driver be able to correctly react by actuation of the hand steering wheel, the gas accelerator pedal and the brake pedal. Such corrective reaction of the driver results from a comparison of stimuli received from speed of travel, transverse and longitudinal accelerations of the vehicle, the force on the hand steering wheel and its rate of accleration. Based on experience with the gripping traction of the road (which is stored in a person's memory), the driver will actuate the hand steering wheel and the brake or gas pedal with some delay. This delay has a two-fold effect. First, the vehicle responds later than desired, after the introduction of an actuating maneuver by the driver, and second, the vehicle continues to respond longer than desired, as for example, after the hand steering wheel is returned to neutral position. Attempts have heretofore been made to minimize the effect of such delay by so-called lead steering systems. In a prior lead steering system, as described in "Transactions of the Society of Automotive Engineers of Japan, Inc.," issue No. 5, June 1973, by Hirao et al, a piston motor is inserted into the mechanical connection between the steering gear and the steered parts of the vehicle. The piston is held by compression springs in a middle position while the cylinder of the motor is connected to the steering gear and the piston to the steered parts of the vehicle. Pressurized fluid is fed through a special control valve to one or the other of the operating chambers of the piston motor in response to rotation of the steering spindle. A pump is arranged on the steering spindle as a sensing component which, depending on the direction of rotation, operates the control valve in such a manner that it continuously feeds pressurized fluid to that operating chamber of the motor effecting a steering movement in the desired direction. In this fashion, the delay in response of the steered parts of the vehicle is at least partially balanced. The elimination of undesirable delay in steering response, is achieved at the expense of interrupting the mechanical connection between the hand steering wheel and the steered parts of the motor vehicle so that the vehicle can no longer be steered in the event of hydraulic system failure.
It is therefore an important object of the present invention to provide a fluid powered steering system which will respond more rapidly to driver reactions to driving and steering conditions. An additional object, in accordance with the foregoing object, is to provide such a fluid powered steering system which avoids interrupting the mechanical connection between the actuating mechanism and the steered parts of the vehicle so that the vehicle remains steerable even in the event of hydraulic system failure.